Chemical and Charge Imbalance Induced by Radionuclide Decay: Effects on Waste Form Structure

Abstract

SUMMARY ExperimentalIn this section, the experimental results for the Radiation Effects task are summarized. Zr + ion implantation at 550 K was performed and Zr and O atomic concentrations of up to 1.5 at% each in STO were achieved. Effects on the crystal structure, chemistry, and charge states in the implanted STO were studied. A number of analytical methods were employed to characterize the implanted STO, including secondary-ion mass spectroscopy, multiaxial ion-channeling analysis, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and Raman spectroscopy. The results show that, in contrast to the observed mobile Sr interstitials in STO, the implanted Zr does not diffuse noticeably during the ion implantation or thermal annealing up to 1423 K. A defect concentration was generated and accumulated in STO during the ion implantation, but the crystal structure was not rendered fully amorphous. Thermal annealing at 1273 K leads to a significant defect recovery at the surface with little recovery occurring at the damage peak, where a modest recovery occurs upon further annealing at 1423 K. The blocking ratio of Zr in STO, as observed along the <001> axis, is ~0.4 and ~0.5 after annealing at 1273 and 1423 K, respectively. Nearly all the implanted Zr species are not located exactly at the original lattice site due to structural distortion. A high density of dislocations was observed in the damage layer, where there were also Zr-containing superlattice structures and voids (or oxygen blisters). Results show that Zr is located at every other Sr site in the superlattice structure. Larger voids and dislocation loops exist beyond the damage layer. There was a minor phase with a tetragonal structure in the damage layer, where the implanted Zr was distributed. The tetragonal phase has the <001> axis parallel to that of the STO host. It survived thermal annealing at 1423 K with only a small decrease in the c value. Changes in chemical states or formation of new chemical bonding were not observed in this study because of the thin implanted layer and limited detection sensitivity. Discussion about the results and a general assessment of the model waste form are also provided in this report.

Topotactic and reconstructive changes at high pressures and temperatures from Cs-natrolite to Cs-hexacelsian

“…Having both Ba-and Cs-HEX structurally well characterized might help to model chemical and structural changes occurring during the decay of 137 Cs + under β and γ emissions to 137 Ba 2+ , changing the cation valence and ionic radius. Further experimental findings in this field have been reported by Jiang and Van Ginhoven (2012).…”